Conventional segment-type filter units comprise an assembly of two or more individual disc segments stacked at their respective hubs on a central common filtrate collection tube within a filter housing. Each filter segment is made up of a central coarse drainage and support mesh sandwiched between layers of filtration and support media and has its filtering surfaces spaced from those of the adjacent filter segments to allow inflow of the fluid to be filtered. Generally, the assembled segment filter unit is enclosed within a housing which is supplied with fluid at a relatively high pressure while the central collection tube is maintained at a lower pressure so that fluid flows from the housing through the filtering surface and exits through the central collection tubes.
Each of the filter segments typically comprises a filtration medium affixed to the surface of a filter support and drainage core which is usually composed of a woven, screen-like wire mesh. The filtration medium may comprise any of many well-known materials, for example, a sintered powdered metal material, sintered fiber metal, finely woven metal or synthetic materials, or polymer membranes. The filter core generally serves the purposes of stiffening the disc, supporting the filtration medium, and providing a porous interior through which the filtrate can drain to the hub after passing through the filtration medium. Alternatively, the filter core may be composed of a porous matrix positively formed by the layering of plates, as described in U.S. Pat. No. 3,702,659 to Raymond C. Clark. The porosity of the core in either case will generally be uniform throughout the interior of the filter segment. Thus, in conventional filter segments during the filtration process, the initial flow velocity through the filtration medium is not uniform over the surface of the filter segments but rather is higher at points closer to the hub than at points more distant from the hub.
As is known by those familiar with the art, non-uniform flow velocity through the filtration medium results in a shorter filter segment life than could be obtained if a uniform flow velocity were maintained at all times over the entire filtering surface, necessitating more frequent removal and cleaning of the filter. Non-uniform flow velocity through the filtration medium also results in a higher residence time, or hold-up time, of some of the filtrate in the segment. This is of particular concern in polymer melt filtration applications, as most polymers experience molecular weight shifts due either to polymerization or degradation when subject to high temperatures. Non-uniformity of residence time will generally result in a lower quality product containing a large range of molecular weights and poorly suited for many manufacturing applications. In some cases, molecular weight can be reduced by 50% in one minute. Where the polymer is prone to cross-linking during exposure to high temperature, increased holdup time can result in the formation of undesirable gels which are very harmful where the polymer resin filtrate is to be cast, or extruded, or blown into fiber, causing optical defects and fiber breakage.
Another important consideration in the design of filter segments is the elimination of "dead spots", or areas of stagnation, where filter fluid, especially polymer melt, can stagnate and decompose during extended exposure to high temperature. In the above reference by Clark, the porous matrix formed by the layering of plates offers opportunities for stagnation in crevices between the layered plates and at the extremities of the apertures of the plates. Such areas of stagnation also interfere with cleaning of the segment to allow repeated use of the filter discs before they must finally be discarded. Areas of stagnation resist penetration of cleaning agents. In polymeric applications, any degraded polymer retained in dead spots after cleaning can catalyze cross-linking and formation of gels when the segment is put back in use.
The importance of minimizing the volume of the flow channels in the filter segments while keeping the pressure drop low is clear. The equations which describe the flow of fluids through channels such as those of the Clark structure, which was discussed in detail beginning at page 18 of this specification, show that pressure drop along the channels varies inversely as the cube of the channel depth. Thus, replacement of the four apertured plates with two parallel paths of flow by a single plate of fourfold thickness will decrease pressure drop by a factor of 32. Such reductions in clean filter disc pressure drop allow greater utilization of the dirt capacity of the filter media without having to go to extremely high changeout pressures, such as 10,000 psi.
Additional problems have been experienced with conventional segment filters when operating under conditions of high pressure, for example, pressures in excess of 1,800 psi. These problems result from distortion of the filtration medium matrix caused by inadequate support of the filtering medium, as explained in the above reference by Clark. Thus, a tradeoff must be made in selecting the core material for a conventional segment filter to provide a core material that has sufficient flow capacity and minimizes internal pressure drop within the filter segment, yet has sufficient rigidity to prevent distortion of the filtration medium. Segment-type disc filters are often exposed to extremely high stresses which can cause permanent distortion of the discs. These stresses result from high pressure drops developed when high viscosity polymer melt flows into the annular spaces between adjacent discs. The pressure drop for flows between parallel plates is inversely proportional to the spacing between plates. Thus a small difference in spacing on opposite sides of a disc can result in significant difference in pressure drop. In practice, variations in disc spacing from nominal may result in pressure differences causing a total force over a disc as great as 1,000 pounds.
To provide additional support to the individual segments of the filter stack and minimize possible distortion of the filter medium matrix, spacers consisting of a hub concentric with those of the filter segments and with radial arms extending outward to the outside diameter of the disc have been used to allow the discs to support one another. Other approaches use a coarse wire mesh between adjacent segments to accomplish this purpose.
In segment designs where a coarse drainage support mesh is sandwiched between identical layers of filtration and support media, edgewise flow through the drainage mesh constitutes a significant resistance to the flow. This high resistance further broadens the range of flow velocities over the segment surface, which adversely affects the dirt capacity of the filter, as discussed above.
Thus, in the design of conventional filter segments, the choice of a supporting mesh is complicated by the conflicting requirements of adequate filtrate drainage through the filter disc and adequate stiffness and integrity, which favor the choice of a coarser mesh, and the requirements of adequate support for the filtration medium and the minimization of the overall thickness of the segment filter discs to allow efficient use of the filter housing space, which encourage the choice of a fine mesh. Conventional filters have utilized a composite of two or more woven wire meshes to achieve both maximum fluid drainage and maximum filtration medium support. Typically, these filters comprise a protective mesh, an exterior filtration medium, a fine mesh support medium, a perforated metal beneath that fine mesh with an open area of about 30-50% to support the mesh and filtering medium, and a core of coarse mesh, typically of 8 to 10 wires per inch, to provide for lateral drainage of the filtrate.
Schemes have been developed to improve the drainage of segment filter discs over that afforded by use of simple wire mesh both to provide drainage and to serve as support for the filtration medium. Such a scheme is described in U.S. Pat. No. 3,702,659 to Raymond C. Clark. However, the benefits of increased drainage area in Clark's concept are accompanied by the high hydraulic area of the apertures in the plurality of discs used to form the segment core of his invention, as well as by the tortuous path the filtrate must travel in passing through that core to get to the segment filter hub. As discussed above, the high hydraulic area of the four apertured plates increases pressure drop by a factor of 32 over that of a single plate. If, alternatively, the four plates are made thick enough to keep the pressure drop within reasonable limits, the resulting increase in the overall thickness of the filter disc will decrease the number of filter segments which can be fitted into a given housing. In polymer melt applications, as discussed above, the matrix formed by such layering of plates offers opportunities for stagnation points in the molten polymer flow and the formation of undesirable gels. These dead spots also interfere with cleaning of the filter segments.
Problems similar to those of high pressure applications may also be experienced in low pressure applications where, for example, membrane filters are used for fine filtration in, as an example, semiconductor wafer manufacturing. As flow density is a function of pore size, large numbers of filter segments are stacked in close proximity to one another to provide the requisitely large filter area in sub-micron filter applications to obtain sufficient flow volumes with low flow densities. A further advantage of utilizing the stack-type filter as opposed to pleated fabric supported filters to attain these large filtering surfaces is the absence of woven or nonwoven supports which might otherwise result in increased air hold-up. Pleated fabric-supported filters utilize fine fiber fabrics of high fiber density to provide adequate support to the thin, low flexural strength filtration media. However, the finer the fiber, the more prone it is to air hold-up and the harder it is to remove trapped air. Air hold-up tends to increase with the fiber surface area of the fabric which is inherently high in fine fiber fabrics of high fiber density. Entrapment of air can be a cause of contamination in certain applications, such as photoresist dispensing, where alternate expansion and contraction of air during successive filtrate dispense cycles can result in loss of control of amounts of photoresist being dispensed.
When membrane materials, such as, for example, Teflon and other polymers, are used for fine filtration, for example, one micron or lower, they must be very thin to keep the pressure drop reasonably low for a given flow rate. These thin membrane materials have a low flexural strength which decreases further with increases in temperature and concomitant decreases in modulus of elasticity of the material. Thus, it is very important to provide adequate support to the membrane to prevent its collapse under the operating pressure while still providing adequate means for drainage of the filtrate and minimizing air entrapment. Devices have been suggested to provide these functions, such as that described in U.S. Pat. No. 4,501,663 by Merrill. In that device, support and drainage are provided by circular channels, concentric about the disc center, which are drained to the central outlet through a number of radial slots. The relatively large distance filtrate must travel along the annular channel before reaching a slot in the areas most distant from the hub results in a greater pressure drop than in a system with more optimized flow distribution. Flow distribution considerations are especially critical in low viscosity fluids, such as, for example, gas flow applications.